1. Field of the Invention.
The present invention relates generally to a method and apparatus for measuring the impulse response of a radio channel, and more specifically, to a digital impulse response probe.
2. Description of Related Art.
Determining the impulse response of a radio channel typically involves correlation techniques. In the traditional pseudo-random noise (PN) probe approach to measuring the impulse response of a radio channel, a maximal length linear shift register generates a pseudo-random noise (PN) code, which is then transmitted repeatedly. The auto-correlation of the PN code is a triangular pulse with a base width of two bit durations. This triangular pulse is the band limited estimate of an ideal impulse. At the receiver, a copy of the transmitted PN code is cross-correlated with the PN code. Since the transmitted (and received) signal is repetitive, the cross-correlation is circular. Therefore, in the impulse response measurement, the delay times of the multipath signals are reduced "modulo" the duration time of the PN code. When multipath transmissions occur on the radio channel, temporal shifts, i.e., delays, occur in the PN codes. These delayed PN codes result in multiple triangular pulses in the cross-correlation. Thus, the cross-correlation is a band-limited estimate of the impulse response of the radio channel.
In traditional implementations, analog signal processing performs the cross-correlation using a sliding correlator method. In this method, a mixer multiplies the received signal and a duplicate of the transmitted signal. A low pass filter or bandpass filter integrates the output of the mixer. The relative delay of the two signals is provided by offsetting the clock rates of the received signal and the duplicate PN code. As the relative delay slips, the impulse response measurement appears at the output of the filter. Some implementations shift the relative delay by occasionally dropping a bit from the duplicate PN code. In either approach, the impulse response is measured sequentially at one delay time after another during the time required for the relative delay to slip through the PN code duration time.